Mass spectrometer



Oct. 23, 1956 z. L. BAY

MASS SPECTROMETER 3 Sheets-Sheet 1 Filed Oct. 26, 1954 5 .QQNIZATION R OT N E V m ZOLTAN L. BAY

ATTORNEYS United States Patent Ofiice Patented Oct. 23, 1956 MASSSPECTROMETER Zoltan L. Bay, Chevy Chase, Md.

Application October 26, 1954, Serial No. 464,789

16 Claims. or. 250-413) particular ion mass accelerated through anelectric field of known potential.

Important scientific applications of the mass spectrometer are: tosurvey the mass composition of a given sample; technical massdeterminations; the determination of molecular weights of complicatedorganic and inorganic compounds; and for gas analysis and thecomposition of hydrocarbon mixtures. For these applications andpurposes, it is desirable to have a mass spectrometer of small size thatis a simple, easy to handle and rugged instrument. The device should becapable of serving for the qualitative detection of the kind of masspresent with speed and convenience, and also be satisfactorily workablefor the more difficult task of quantitative mass determinations.

Production and the control of the ion flight within the spectrometerenvelope and the detection of ions arriving at the receiving electrodehave posed ditficult problems. Ions may be produced by electrons emittedby a hot cathode electrode or by infrared and visible radiations andthese electron or radiation fields should have no appreciable effect onthe ion detection means. Electron and ion paths within the spectrometermay be separated by combined electric and magnetic fields. But animportant feature of this invention eliminates need for a magnetic fieldor magnetic apparatus. According to the invention, ions are detected bythe electric pulses produced on their arrival at the plate of thespectrometer, the pulses produced by different ion species beingselectively gated by means having a determined time delay with respectto the initiation of the ion travel to the plate. By selectively varyingthe gating delay times, only the ions corresponding to definitecomponents of the compound will be detected, while the relativeamplitudes of the gated pulses produced by these ions give an indicationof the relative quantities of the different ion masses. This gating maybe accomplished either internally of the spectrometer envelope orexternally. When internal gating is used, ions are detected by secondaryelectrons given off from the plate electrode of the spectrometer by theimpact of ions and an electrostatic barrier field is arranged toefiectively keep ion generating electrons on one side and secondaryelectrons on the other side. One form of external pulse gating will beshown in which a pulse coincidence technique is used to gate only pulsesof selected time delay subsequent to initiation of the ion burst.

Other ideas of this invention serving to improve the action andperformance of a spectrometer may be considered.

Combining the ideas of generating ions in bursts of very short timeduration with short ion flight times and small flight distances servesto increase the resolving power of a spectrometer and differentiatebetween ions having masses within one percent of each other. The veryshort electric pulses which are necessary for the production of burstsof very short duration may be generated by pulse techniques andapparatus understood and used in the electronics field. Short ion flighttimes and distances have an important advantage in that any desiredpotential distribution along the path of ions can be obtained with goodaccuracy and a consequent efiectiveness of the mass reso lution.Further, intense ion beams can be obtained to give a high order ofsensitivity to the spectrometer.

In accordance with the invention, measurement of short ion flight timeshaving a high degree of accuracy has been found possible by a delayedcoincidence method. Here, short voltage pulses which control the ionsource are used again after a predetermined time delay to register theions arriving at the plate electrode of the spectrometer. Only such ionsas have a flight time coinciding with the pulse delay time willregister. Since flight time is dependent upon the ion mass andaccelerating voltage, this accelerating voltage can be controlled orvaried as required for ions having differing masses.

For best measurement accuracy, ions should be time focused or behave asif they all start from a fixed point or area, which may be an innercylindrical surface in a spectrometer of convenient cylindrical symmetryand con struction. A close approximation of such a focus effect may berealized in practice by simulating a field of parabolic potentialdistribution: in such a field, the ion time of flight between any pointand the focus point is constant and ions arrive at the focus point atthe same time independent of actual ion originating points. A goodapproximation of a field of a parabolic potential distribution may beset up within a spectrometer envelope by the use of a grid electrodeproperly spaced and biased.

In view of these general ideas and features pertinent to a time offlight mass spectrometer design, the objects of this invention are:

To provide a mass spectrometer design using short times of ion flight indimensions of small space;

To provide a design adapted for a measuring method using secondaryelectrons;

To provide a simple time measurement means;

To provide a means for time focusing ion beams;

To provide a design having cylindrical symmetry adapted for easyconstruction by present radio tube manufacturing techniques;

To provide a construction having electrical elements adapted foroperation by simple electronic circuits;

To provide detail improvements increasing accuracy and resolving powerof a spectrometer.

Further objects of this invention will be understood from the followingspecification considered with relation to the drawings.

In the drawings:

Fig. 1 is a vertical sectional view of a mass spectrometer simplified toshow essential ion control elements of this invention;

Fig. 2 is a sectional view taken along the line 22 of Fig. 1;

Fig. 3 is a schematic diagram of a pentode electron ion generatorsuitable for construction within the inner portion of the massspectrometer of Figs. 1 and 2;

Fig. 4 is a diagram of voltages representative of those applied to theion and electron control elements or electrodes of the spectrometer ofFigs. 1 and 2;

Fig. 5 is an enlargement of a part of the diagram of Fig. 4;

Fig. 6 is a diagram of apparatus for operating the spectrometer of Figs.1 and 2;

Fig. 7 is a diagram illustrating a form of the time focusing idea ofthis invention;

Fig. 8 is a voltage diagram similar to that of Figs. 4 and 5 but for aspectrometer having fewer electrodes;

Fig. 9 is a schematic sectional view of a structure combining theelements of Figs. 1 and 3;

Fig. 10 is a schematic diagram of an arrangement corresponding to Fig.8;

Fig. 11 is an idealized graphical representatipn of the results of atypical analysis; i

'Fig. 12 is a schematic diagram illustrating the principle of externalgating; and

Fig. 13 is a circuit diagram, corresponding to 6, but employing externalgating.

With reference to'Figs. 1, 2 and 9 of the drawings, 1 is thespectrometer envelope or tube which may be made of glass and, in atypical embodiment, may have, a diameter of 7 centimeters andalngth of 5centimeters. Details of construction of the spectrometer tube willclosely accord with practices commonly employed the manufacture of radiotubes wherein a pluralityof grids, a plate and a cathode having a heaterare mountedwithin an envelope which can be evacuated. Constr uction ofthis spectrometer tube differs from that of a ra 0. tube primarily inthat: the spectrometer requires. an opening 2.

which can be closed or opened readily for introduction and removal oftest samples; and, it is desirable to provide a ground glass or othersuitable impernianent seal 3 to permit replacement of a cathode or othercentral elements of the spectrometer in case of failurewithoutdestruction of the major part of the structure. Functioning electrodesof the.spectrometenexeept, for

the ion generating source, include grids 4,5, 6 and 15 and plate 7 whichare suitably spaced and supported as by mica discs 8 and 9 and haveelectrodeeleads (not:

shown) for external electrical connections brought through the base ofthe spectrometer tube. These grids and plate are coaxial and ofcylindrical shape and are accurately spaced as will be explainedhereinafter indetail. Plate cylinder 7 preferably has a diameter of 6.4centimeters and grid 4 may have a diameter off2 centimeters. Exactlength of the cylindrical electrodes 4-7 inclusive is not critical andthe proposed length here is about 4 centimeters.

The ion generating source of the spectrometer is essentially an electrontube such as the pentode structure represented schematically in Fig. 3.The only changes in the ion generating source from the construction 'ofa typical electron or radio tube are: plate 4 of Fig. 3 is made in theform of a grid to permit electrons to pass therethrough; and part of thestructure including at least the heater and cathode is preferably maderemovable and replaceable in case of failure. This ion generating sourcemay be a pentode tube structure with elements including a cathode 10, aheater 11, a control grid 12, a screen 13, a suppressor grid 14, and ascreen plate 4 which is idcn tical with grid 4 of Figsfl and 2. All or"these elements should have a cylindrical symmetry. Replaceable parts ofthis pentrode electron tube structure may be based on a suitable groundglass plug fitting the ground glass seal 3 of envelope 1 of Figs. 1 and2.

Operation of the electron tube of Fig. 3 is effected by means of asuitable power supply apparatus and circuits of a type familiar inelectronic equipment. Provision is made for pulsing control grid 12 by acircuit connection with a pulse generator as shown in Fig. 6. Details ofthe pulse generator and circuitry of the ion generator of Fig. 3 are notof immediate concern in this invention since these matters are wellunderstood in the electronic field. For the purposes of this invention,the ion generator must be capable of delivering short ionizing electronpulses of a time duration of the order of 10 second. Sufiicient ionizingelectrons may be delivered at a current of 1 to 30 milliamperes which iswithin the plate current range of a pentode tube structure similar tothat of the familiar radio tube known as the type 6AC7.

With the insertion of the ion generating means of Fig. 3

in the spectrometer structure of Figs. 1 and 2 as shown in Fig. 9,essential operating elements of the spectrometer device of thisinvention will be completed. Further consideration will be given to thepurpose and function of individual elements, their relative size orspacing, relative applied potentials, and other details or accessoryapparatus required to make the spectrometer serve effectively for itsintended purposes.

Desirable small size in a spectrometer device is possible only where theion flight distance is short and of the order of a few centimeters inlength. For satisfactory resolution or the ability to recognize ionshaving differing mass numbers, the time duration of ion bursts. must bequite short relative to the ion flight time over the ion flightdistance. Ion bursts of a very short time duration can be produced bysimilarly short ionizing electron bursts produced by suitable pulsesapplied to the control grid of the electronic ion generator. Means forproducing pulses of a very short time duration havebeen developed in theelectronic field apdparticularlyinthe: technique of photo multipliertubes. For the purposes of this invention, pulses, electron bursts andion bursts preferably have a time dura tion of, theorder, of 10- second.Such a pulse gcperatpr isschematically shown at 22 in Fig. 6. Any.suitable repetition rate of thesepulses, for example, 10 p l e e ec nd my e sqd,

In the spectrometer of Fig. 9, ions are accelerated through a pote ntialappliedhetween grid 4 and plate 7 and theLiqn flight distance is thedistance from grid 4 tol plate 7 which may have a length of 2.2centimeters.

Ions having a mass M and an energy eVo (where ais.

When time t, distance L and the potential V0 are known or measured, themass M can be determined.

For a preferred operating simplicity as atforded by application of theideas of this invention, distance L of (l) is fixed by the positions ofgrid 4 and plate 7; and time t is also fixedby a time delay coincidencemethod whereby the ion.detecting means is activated by a pulse of fixedtime delay following the ion generating pulse-as shown by pulsedelaynetwork 24. in Fig. 6. Potential V0 is readily controlled and measuredand the determination of ion mass is then. simplifiedto a measurement interms of the value of potential Va.

Ion and electron control elements or electrodes 4, 5, 6,. 15 and 7 ofFigs. 1 and 2 have applied potentials as indicated in Figs. 4 and 5 andthese applied potentials or voltagesarederived from a suitable powersupply unit of conventional type schematically indicated at 26 in'Fig.6. In theinterest of clarity, grids 13 and 14 are omitted inFig. 6,. astheir action is conventional. In view ofthe electric fieldscreated bythese applied potentials, ionizing electrons from the generator withingrid 4 are forced back, by thefield between .grids4 and 5 whilethe ionsare accelerated toward electrodes. 5 and 7. The important ionizingregion, of the mass spectrometer is in the spacebetween grids 4 and 5.and quite close to grid 4. Grid 5 and plate .7 are. operated at-apredetermined fixed potential difference.

Between electrodes 5 and 7 are grids 6 and 15; grid 6 serves as acollector of secondary electrons ejected by ion impacts on plate 7 whilegrid 15 serves as a time controlled gating grid for the passage ofsecondary electrons from plate 7 to grid 6. Grid 15 receives a delayedpulse voltage Pd in addition to the direct current potential indicatedin Fig. 5. Since grid 15 has a steady potential normally negative withrespect to the potential of plate 7, secondary electrons can passthrough grid 15 only when the pulse voltage Pd raises the potential ofgrid 15.as, for example, from 2l5 volts to 12.5 volts. Secondaryelectrons from plate 7 cannot enter the important ionizing regionbetween grids 4 and 5 of the spectrometer because grid 5 is at a lowpotential relative to the potential of grid 6. As an example, relativepositive potentials of electrodes 6, 15 and 7 may be respectively 15volts, 2.5 volts and volts above the potential of grid 5 which is at azero reference potential. A comparatively high positive potential of afew hundred volts may be applied to grid 4.

As will be apparent from Fig. 6, production of ions occurs in very briefpulses whose timing is determined by operation of pulse generator 22,these pulses also appear, after a suitable delay determined by thecharacteristics of network 24, at grid 15, and raise its potential tothe necessary gating level. If, at that time, secondary emissionelectrons have been produced by arrival at 7 of the ions released by thesame pulse from the ion generator, then a charge appears at collectorelectrode 6 due to the arrival of the gated secondary emission electronsfrom plate 7 through pulse gating grid to collector electrode 6, thedistributed capacity of which is represented by a dash-line condenser16. The charge suitably amplified at 28 is impressed on oscilloscope 30.If the accelerating voltage between screen plate 4 (acting as cathode ofthe mass spectroscope) and plate 7 is modulated cyclically by the outputof source 32, which may be a 60 cycle source, then for every particularmass of ion in the field the corresponding voltage will be reached atsome phase of each cycle, which is necessary to give it just thatacceleration which will cause secondary emission due to the ions of thatmass reaching plate 7 at the correct time to pass gating grid 15, and soa pulse will be periodically reproduced for that mass at a particularpoint in the sweep of the oscilloscope. The resulting trace on the scopefor a gas containing ions of three masses, A, B, and C'respectively,will appear as in Fig. 11, the relative heights of the respective pulsesbeing related to the proportions of the respective quantities.

It will be apparent that vwhile a cathode ray oscilloscope is shown forregistering the results, any other known means for measuring the transittimes as related to the accelerating voltage may be employed. Forexample, if the device is used for testing for the presence or absenceof a single element, as in a leak detector, then the system canobviously be simplified; a fixed accelerating voltage would then beemployed as required for that particular element, and so related to thedelay that only when that element is present is a pulse gated through.The oscilloscope may then be dispensed with and a simple pulse detectorassociated with grid 6 is sufiicient for the purpose. However, themethod is fundamentally the same in both cases and it will be clear thatmany other variations'in the application of this method of determiningthe mass of ions can be employed as required by different types ofresearch. It is also clear that instead of cyclic variation of theaccelerated voltage, this could be varied manually in small steps and areading taken at each step. The variable parameter need not be theaccelerating voltage, as shown by way of example, but could be the delaytime, as the delay can obviously by varied either cyclically or indefinite steps while maintaining the accelerating voltage constant, toderive the same information.

An alternative arrangement for time controlling the passage of secondaryelectrons from plate 7 to grid 6 is represented in the voltage diagramof Fig. 8. Here the gating grid 15 is eliminated and plate 7 itselfreceives a delayed pulse voltage Pa of a value sufficient to lower thepotential of plate 7 with respect to the potential of grid 6. When plate7 is pulsed and grid 6 is at a higher potential, secondary electronsemitted from plate 7 are collected by grid 6. This alternativearrangement for controlling secondary electrons may be employed in aspectrometer tube design where the relative radii of the ion focusingelements are such as to make the space between grid 6 and plate 7 quitesmall. As a further alternative, in the arrangement of Fig. 8, thepotential of 6'. plate 7 may be maintained constant and grid 6 would bepositively pulsed to attract secondary electrons emitted from plate 7.

By reason of the pulse timing coordination employed in this inventionwhere a pulse Pi is applied to the ion generator and is delayed toappear as a delayed pulse Pd applied to the secondary electron sensingelements 6 of this spectrometer, it will be clear that only ions of aparticular mass number will be registered on the oscilloscopes screenfor a definite value of the ion accelerating voltage V0. Thisregistration is accomplished by detecting the arrival of secondaryelectrons at grid 6, which produces a negative voltage pulse at thisgrid. Ions must travel the fixed flight distance in the fixed delay timet in order to register at the secondary electron collector 6 which isactivated only at time intervals t. Where ion generating pulses having atime duration of l0 second are employed, these pulses may be repeated ata rate of 10 times per second. Ions of dilfering masses can beregistered by changing the ion accelerating voltage V0. Sinceaccelerating potential V0 must be changed between very great limits inorder to scan a large mass range, the delay time t may be varied insuitable steps as by using any one of the steps t1, I2, is tn. Thesetimes t1, t2, etc. would of course be selected to correspond to thedifferent masses which are being investigated, that is, they wouldcorrespond to the ratios of the masses of the successive elements.

Continuous operation of the time of flight mass spectrometer of thisinvention is effected by adding to the direct current acceleratingpotential V0 an alternating current potential Vt, which can be 60 cyclesper second. Potentials V0 and Vt combine to give a slowly changingaccelerating potential Va. which will be considered with relation to thepulses P1 and Pa which may be repeated at the rate of 10 per second ormore. For every mass M1; there exists a definite voltage interval Vk--Vk of the accelerating potential Va Within which secondary electronsejected from plate 7 by ions of mass Mk arrive at collector grid 6. Thepotential Va will be within this potential interval during a timeinterval llr-l'k. If the total variation of potential of Va covers, forexample, a mass range of 20 mass numbers as from M to M plus 20, thepotential Va is in the voltage interval Vi V1; for approximately of thecycle period of of a second. During this short period or" about V1200part of a second there are from 10 -10 pulses. This large number ofpulses in a short time interval permits secondary electron collectorgrid 6 to pick up a relatively large number of secondary electrons witha resulting sizable value of signal current and an important improvementin the effective signal/noise ratio of the measurement.

Essential circuitry and accessory apparatus required for the operationof the spectrometer of this invention are represented in the simplifieddiagram of Fig. 6. A pulse generator of a suitable type delivers a pulsePg to the electronic ion generator of the spectrometer and a similarpulse Pg to a delay network which may be a coaxial cable and the outputpulse Pd is applied to grid 15 of the spec trometer through a capacitor18. Resistor 19 supplies grid 15 with a potential as indicated in Fig. 5from a suitable power unit which also supplies by conventional circuitryother electrodes of the spectrometer with required operating voltagesand power. Grid 6, which is the collector of secondary electrons as hasbeen described hereinbefore, is coupled with an amplifier whose outputis connected with an oscilloscope for visual indications.

The charge of secondary electrons delivered by 10 -10 pulse periodsreceived by grid 6 is discharged through resistor 17 where resistor 17with capacitor 16, which represents the distributed capacitance of grid6 may have a time constant 10 second. Since this time constant is in theaudio frequency range, the pulses appearing at grid 6 as a result of thecumulative pickup and discharge of secondary electrons are readilyamplified by an audio amplifierhaving a. cut-otf at a frequency whichmay be as low as. 1000 cycles per; second. Ionizing pulses P and delaypulses Pa repeated at the rate of 10 timesper; second, will not bepassed by such an audio amplifier, but the pulses of secondary electronsgroup collected during a time interval such as tk-tk corresponding tomasses Ms present, in the spectrometer. will beamplified fora showing onthe oscilloscope or other indicator. By applying the alternatingcomponent Vt of the ion accelerating voltage Va as a sweep to anoscilloscope, different masses present in the spectrometer arerepresented separately on the screen by deflections proportional to thedifferent ion mass concentrations in the gas sample under analysis.

It will be clear, then, that the method and means for registering ionsby collection of secondary electrons has much latitude and practicaladaptability. The cumulative charge of the collector grid and itsdischarge through an RC circuit gives voltage pulses of appreciablemagnitude with a practical elimination of serious ditliculties associated with the handling of otherwise weak voltage changes both within thespectrometer itself and outside the spectrometer in the amplifier andindicator. No magnetic fields are needed. An audio amplifier of simpledesign and a standard type of oscilloscope serve satisfactorily foramplifying and indicating purposes. This audio frequency type ofmeasurement eliminates compensating difficulties inherent in a directcurrent type of measurement such as: need for very stable voltageconditions and their repeated checking; and need for compensating thecurrent of grid 6 for current caused by ions striking it and for thesecondary electrons striking grids 5 and 15. Also, the audio type ofmeasurement does not make it necessary to keep background current at aminimum as may limit grid area to using thin wires widely spaced.Limited grid areas give a poor electric field distribution detrimentalto spectrometer performance.

The preceding description shows the manner of using internal gating todetect the pulses. It is also possible to detect the pulses by externalgating, as will be shown below. This has the advantage of requiring asimpler tube construction, since secondary emission gating is not used,and, therefore, electrodes 6 and 15 of Fig. 9 can be eliminated; exceptfor this the tube may be as shown in Fig. 9; alternatively, a lineartube construction may be employed. The detection will in this case takeplace directly at the plate 7, which is connected to the external gatingcircuit.

The original ion pulse appears at the plate as a group of n pulses,where n is the number of components of the mixture. All of these 11pulses appear within a short time which is a fraction of the time offlight of the slowest ions of the mixture. It is clear that afterindicating one of the 11 pulses the indicating device must rapidly comeback to its zero position in order to be able to indicate separately thenext ion pulse of the group. This requires an indicating device capableof high speed operations. Let the pulse period of the original ion pulsebe denoted by T, then T is also the period of each of the separated ionpulses of the group arriving at the plate and the speed of operation ofthis indicating device must be such as to resolve signals separated bythe time T or smaller. This means in the well known language of radiotechnique that the band width of the indicating device must be at leastin the order l/T sec- On the other hand, in the new method, a gating isapplied which is elfective only for one pulse of the group of 11 pulsesand, therefore, the indicating device is not required to come back toits zero position rapidly after accepting the signal because in thismethod there is no need to indicate the next pulse of the group. Thismeans that the output pulse of the indicating device can be severalorders of magnitude longer than the input ion pulse and the frequencyband width can be chosen several orders of magnitude smaller than l/T.

,- tional to the pressure.

Asis:we1l.kn,own;. the inherent noise of any indicating device:(including. amplifiers, instruments, oscilloscopes, etc.)is-.pr.oportional to the square root of the band width of the device.lower bandwidth for the detecting device which is equivalenttto abetter'signal to noise ratio. Since the signal to noise ratiodeterminesthe lower limit ofsensitivity, it is clear'thatrnuch weakersignals can be detected with the method of the invention. In thelanguage of'mass spectrometry, this means that much weaker components ofa gaseous mixture canbe detected, even though the detecting device is lsimpler and cheaper.

Another advantage-of the new method is that by repeating the originalion pulses the long time constant of the indicating deviceproduces acomulative effect. This increases the signal to noise ratio further andthis increase'can amount to several orders of magnitude.

Another advantage of the new method is that the pulse period and timeconstant of the indicating device are independent quantities. This meansthat T can be chosen as small as permitted by the limits of known pulsetechniques. (e. g., T=l0- sec. or less), while the amplifiers usedin theindicating can operate at, e. g., audio frequencies.

Another advantage of the new method is that, choosing T very small, thedistance of flight of the ions can be very small1(e. g., if*T=1O' sec.,the distance of flight can be as small as approximately one cm.). Itshould be noted that decreasing the distance of flight, i. e.,decreasing the sizeof amass spectrometer, is not only a matter ofconvenience. The number of collisions between ions and neutral atoms ofthe gas inside the spectrometer. is proportionalto the path length andpropor- Thus:a small path affords the possibility of operating athigher. pressures and thus detecting weaker components.

The methodillustrated by Figs. 1-10 utilize the gating ofsecondaryelectrons released by the impact of ions. In the following, description,there is described the useof external coincidence circuits for gatingthe ion pulses. Fig. 12 shows-the general blockdiagram of the equipmentfor utilizing the new method. P is the plate of the mass spectrometertube at which the ion pulses arrive, correspondingto plate 7 of thepreceding figures. P is connected through channel A to a coincidencedevice. The

pulses from the pulser, (which is not indicated in the diagram) travelthrough channel B and, after a properly chosen time delay, arrive at.the input of the coincidence circuit. The coincidence device is acircuit which gives an output only if two pulses, appear simultaneouslyin the individual channels A and B. The circuit can be constructed insuch a way that the amplitude of the output is proportional to theproduct of the amplitudes of the coinciding pulses in the channels A andB. Since the pulses of channel B are always of the same amplitude, theoutput pulses of the coincidence device is proportional to the amplitudeof A, i. e., the number of ions arriving at the plate in coincidencewith the measuring delayed pulse in B. Channels A and B are constructedso as to have a very broad frequency response which can extend farbeyond 1/1". A suitable method is to use concentric cables of very smallattenuation up to frequencies 10 secf At the input of the coincidencedevice the cables are matched, to give no reflections. It is easy todesign the inputs of the coincidence device-to have time constants ofthe order 10" sec. or less, so no appreciable memory of the pulsesremains in the input after a time interval of 10- sec. The output of thecoincidence device can easily be constructed to have a time constant ofunlimited length, e. g., 10- sec., even up. to several seconds. Inconsequence of this, the output pulses can be amplified by means ofaudio amplifiers of very narrow band width, The measuring or indicatinginstrument can The use of the new method: results in a.

greases 9. be an oscilloscope or any current or voltage measuringdevice.

In order to obtain the entire mass spectrum of the mixture to beanalyzed, the measurement is performed stepwise as previously described,changing either the accelerating voltage inside the mass spectrometertube or changing the time delay of the measuring (gating) pulses inchannel B and relating the mass of ions to the varied parameter.

The coincidence device in Fig. 12 can be any type of known circuitscalled in the literature, coincidence circuits or gating circuits. Thespecial problem in connection with the present invention is to adaptsuch a circuit to indicate very small pulses, because great sensitivityis required of the mass spectrometer. A special circuit developed forthat purpose is shown in Fig. 13.

In the circuit of Fig. 13, 41 and 42 are shielded cables leading to thetwo respective inputs of the coincidence circuit. Resistors 43, 44 and45 are the so-called matching resistors chosen so as to avoidreflection. 46 and 47 are rectifier diodes (crystal diodes or tubediodes), the arrows indicating the direction of positive current throughthem. All the parts, 41-51 together, compose the fast part of thecircuit capable of accepting and handling very short pulses if theelements are properly chosen. The magnitude of the time constant of thispart of the circuit is the product of the matching resistance and thecapacity of the diodes, e. g., if the matching resistance is in theorder of 100 ohms and the capacitance of the diodes in the order of 1,u.L f., the time constant is sec. It should be noted that the condensers4851, the capacity of which can be 10-100p41. f., do not contribute tothe time constant because neither side of them is grounded.

The parts 52-60 on one side [and 61-69 on the otherside represent theslow parts of the circuit. Both sides are similar and either of them canbe used separately or in combination. In the description of theoperation of the device, only one of them, the parts of which arenumbered 52-60, is discussed in connection with the fast part of thecircuit.

The delayed gating pulses of the pulser appear in cable 41, the ionpulses of the plate 7 of the mass spectrometer appear in cable 42. Tounderstand the operation of the device assume first that there arepulses in cable 41 but no pulses in cable 42. In this case point a is atground potential, the same as point b. In this case, the branchesconsisting of 48, 46, 49, 44 on the one hand and 50, 47, 51, 45 on theother hand are identical and thus a short positive pulse in cable 41causes the same amount of current through the diodes 46 and 47. Theseshort current pulses through 46 and 47 charge the condensers 49 and 51positively and the condensers 48 and negatively and if the diodes andthe condensers are similar, all the charges will be of the same amount.

After the decay of the supposedly short gating pulse the condenserscannot discharge through the diodes because of the opposite polarity.Thus the charges flow over to the slow parts of the circuit. Considernow the lower part of the circuit diagram. The negative charge of. 48flows through 52 to the RC circuit consisting of condenser 54 andresistor 56. Similarly, the positive charge of 51 flows through 53 tothe RC circuit 55, 57. The time constants of these RC circuits are equaland they can be very long with respect to the pulse period of the gatingpulses.

Assuming equal charges coming from 48 and 51 and equal time constants ofthe RC circuits, the voltages across the condensers 54 and 55 are equal,but of the opposite sign, during the entire discharging period of thecircuit. This means that connecting the upper sides of 54 and 55 throughthe equal resistors 58 and 59, the common point of 58 and 59 remains'atzero poment of each spectral line.

tential during the entire discharging period. This point is consideredas the output of the coincidence circuit and is connected by line 7% tothe oscilloscope. It is possible that the characteristics of diodes 46and 47 are not exactly equal in which case the said charges differ by asmall amount. To balance the output 60, in the balance positionpotentiometers provided for 56 and 57 and 58 and 59 are connected to themoving taps achieving hereby the balancing of the output. Consequently,after balancing the circuit no output pulse appears, if there are pulsesonly in cable 41.

It is easy to see that there will be also no output pulses, if there arepulses only in cable 42. Assuming these pulses are positive, diode 46will not conduct; therefore, no charges will be generated in thecircuit.

It is also easy to see that in the case of coincidence there will be anoutput pulse because the voltage across diode 46 will differ from thevoltage across diode 47; therefore, different charges will be generatedin 48 and 51. The amplitude of the output pulses is, in this case,proportional to the pulse amplitude appearing in cable 42 and its periodwill be given by the time constant of the RC circuits 54, 56 and 55, 57.The resistors 52 and 53 are chosen to be one order of magnitude smallerthan 56 and 57, and the resistors 58 and 59 one order of magnitudelarger than 56 and 57.

The slow circuit 6169 in the upper part of Fig. 13 is built in a similarway to the lower part to assure symmetrical operation of the circuit.Its output, 69, gives pulses of the opposite polarity of the output 60.

Both of the outputs 60 and 69 can be used separately or they can beadded after reversing the sign of one of them in the later amplifyingprocess, as shown in Fig. 13.

It is clear that the amplifiers used to amplify the output pulses do notneed to have a broader band width than the order of magnitude of l/RCseci. e., in the new method it is not necessary to use high speedamplifiers. It should be pointed out again that this is an essentialfeature of the invention and not only a matter of convenience. It meansthat small signals, i. e., a small number of ions, which are below thenoise level of the high speed amplification process and which thereforeare entirely undetectable with the old method, become detectable by themethod and apparatus of the invention.

It should also be noted that there is no upper limit in the choice ofthe RC time constant and therefore no theoretical lower limit of thecharge (number of ions) detectable with the new method.

A further advantage of the invention is the possibility of accumulationof signal charge when applied to the method of mass spectroscopy inwhich the ionizing pulses are repeated in time. It is clear from theabove description of the operation of the coincidence circuit that theactions of all of the selected ion pulses which appear Within the timeRC, will be added in the output. The time of accumulation is not evenrestricted to the RC time constant of the coincidence circuit proper.After amplification in the frequency range extending up to l/RCseerectification and prolongation of the pulse to a much greater timeperiod can be effected and in this case the accumulation is effectivewithin this much greater time interval. The time of this prolongation istheoretically limitless and it depends only on the time the experimenterwants to spend on the measure- It can be, e. g., 10 sec. in the case ofscanning the mass spectrum on an oscilloscope, as described above; itcan be extended to many seconds or minutes in the case of D. C.measurement of extremely weak components. It should be noted that thisis not true of the methods of measurement shown in the prior art wherethe indicating device must come back to its zero position Within thetime separation of the pulses in order to be able to indicate the nextion pulse.

In addition to the accuracy of the time measurement factor, theperformance and realized accuracy of the complete spectrometer systemdepends also on correctly knowing the value of the ion acceleratingpotential and the precise construction of the spectrometer tube. Knownpulse techniques together with the ion registering method and apparatushereinbefore described are adapted to provide a time factor ofsatisfactory accuracy. Familiar voltage supply apparatus or powersupplies for electronic apparatus are adequate to correctly provide anion accelerating potential of any desired type. The direct precisionrequired in the construction of the spectrometer tube does not exceedthat employed in the manufacture of radio tubes; in fact, thespectrometer tube may be made in substantially the same way as a radiotube having either a glass or metal shell. But in a time of flight massspectrometer tube, consideration must be given to the fact that ions aregenerated in a region rather than in a point generated line or surface.Therefore, the exact flight distance of ions cannot accurately be takenas the distance between two electrodes having a point generated line orsurface configuration; In an ideal spectrometer tube of cylindricalconstruction, the ion flight distance would be the distance between twocoaxial cylindrical electrodes of zero thickness.

A time focusing idea of this invention is practically applied to makeion time of flight distances accurately uniform even though the ionoriginating points do not conform with ideal conditions. This ionfocusing scheme may be explained with reference to Fig. 7 which issimplified to showing of electrodes 4, and 7 only of the spectrometer ofFig. l. The ideal ion flight distance would be the distance from grid 4to plate 7; but, since ions are generated in the shaded region 20 ofFig. 7, ideal conditions cannot be realized. However, if ions lo, 11,and I: of the same mass but originating at different points in region 26are considered, it will be clear that the-v can have the same flighttimes if a compensating difference in accelerating potential can begiven to each ion. For example, if ion 10 starts from grid 4, ion 11starts from the center of region 20, and 12 starts from' the outer edgeof region 29, respective decreasing accelerating potentials V0, V1, andV3 can serve to equalize the ion flight times over their respectivedistances from points of origin to plate 7. All ions of equal massoriginating anywhere in region then are time focused to have the sametime of flight and distance relation of an ideal ion It) which travelsthe exact distance from grid 4 to plate 7.

Consider ions of the same mass M, which are accelerated in an electricfield of the potential V(x). They start with zero velocity at differentpoints of the coordinate x5 (x5 is variable) at the same time t=0 andarrive at the coordinate xii (x3, is the same for all ions). The time offiight over the distance Jar-x8 is t(xs) =const. The mathematicalsolution of this problem is the parabolic potential function, i. e.

which is independent of 2:5.

To provide such an electric field a uniform space charge would be neededtherefore it can not be realized in vacuum. The idea of this inventionis that such an electric field can be approximated by the application ofa few grids (instead of the space charges), or with only one gridbetween as and Xa and high accuracy can be maintained if (instead of theexact solution) we look for the solution of the following equation:

Having fulfilled this condition, the time of flight will be constantwith a high degree of accuracy. Where the starting zone of ions is arelatively small part of the flight distance, the flight time will besubstantially constant, or

t(xs) :const.

The simplest arrangement for the realization of time focusing is thatrepresented in Fig. 7 where a homogeneous electric field is producedbetween grids 4 and 5 by an ion accelerating potential V0 and a field offree space exists between grid 5 and plate 7. Simple calculations showthat the distance between 5 and 7 should be twice the distance between 4and 5 to fulfill the condition It is true that grids 6 and 15 of thespectrometer of Fig. 1 are in this space between electrodes 5 and 7, andelectrodes 6, 15 and 7 have small applied potentials relative toaccelerating potential V0; but, these differences from the simplestarrangement have only a small order effect requiring no more than aminor change in electrode spacings. The major feature for therealization of satisfactory time focusing is the production of anelectric field between grid 4 and plate 7 which closely approximates theeffect of a field of parabolic potential distribution. The potential Vat any point in a field of parabolic potential distribution between twoelectrodes increases proportionally with the square of the distance fromthe electrode which is at the lower potential.

In the case of an electrode arrangement having cylindrical symmetry, thefield and potential distribution between time focusing elements of thespectrometer differ from the parallel plane condition of Fig. 7. Herethe time focusing is already partly achieved by the fact that ahyperbolic field between cylindrical grids is itself an approximation ofthe parabolic field. Therefore, this condition dt dz,

Using also grid 5 (out of other reasons to obtain secondaries in a fieldfree space as released by ion impact at 7) the distance between 5 and 7should be less than twice of the distance between 4 and 5 which utilizesthe focusing effect of the hyperbolic field. The relative radii ofcylindrical elements 4 and 5, which may be selected as desired within anoperative radii ratio range, determine the focus area and the radius ofplate 7.

It is convenient herein to give tables of representative dimensionswhich have been derived for the time focusing elements of thespectrometer of this invention. If three concentric cylindrical elements4, 5 and 7 are used for time focusing, it will be clear that ionsoriginating in an area close to element 4 will be accelerated towardandthrough grid 5 and will arrive in time focus at plate 7. Ions areaccelerated in the space between grids 4 and 5 but travel the remainingdistance to plate 7 in free flight. For time focusing, then, the ionfree flight distance 13 isproportioned with relation to the ratio of theradii selected for grids 4 and 5.

Where .7'4 is the radius of grid 4 of Figs. 1 and 2, m is the radius ofgrid 5, and r7 is the radius of plate 7, a first case, Table I,represents a design employing a fixed distance d(45) between grids 4 and5, while a second case, Table II, represents a design wherein the ratioof radii ra/r-i' is a constant which is here made 3.

Table l Cm. 0111. CD1.

Table II n tin-) r5 11(5-7) T7 001-7 0m. CD1. CD1.

In Tables I and II, the radii of elements 4, 5 and 7 are given togetherwith the distances d(4-5), d(5-7) and d (4-7) between respectiveelements. It will be observed that in Table I the ratio r4/r5 has threediflferent values and tables similar to Table II representing only achange in size is readily worked out for these differing ratio values.For three electrode focusing, the ratio r5/r4 may be selected in thevalue range between 1 and 3.55. Where the ratio is equal to 3.55,elements 5 and 7 coincide and time focusing occurs between only twocylinders.

There is, then, much engineering latitude allowable in the applicationof the time focusing principle of this invention. Well known in ionsources of small dimensions generate ions of too small a number for goodresults in a time of flight mass spectrometer. focusing, spectrometeraccuracy is not limited to the use of a small ion generating source andthe inner cylindrical element 4 of Figs; land 2 is readily made largeenough to contain electronic means of sufficient power for thegeneration of ions in great number.

It will be understood that complications arise in the operation of aspectrometer device which set a natural limit to the accuracy ofmeasurements. Inaccuracies arise from the thermal agitation and are thesame for all ions independent of mass but will be under 1 percent asrepresented by: errors of 0.8%, 0.4% and 0.25% for, respectively, 100,400 and 1000 volt values for the ion accelerating voltage V0. The spreadof initial energy caused by ionizing electrons is negligibly smallexcept for ions of very small mass where mass differences are relativelygreat and separation accuracies are unaffected. In view of these andother effects understood in the technique of mass spectrometry, someinaccuracies will exist but can be kept small or well within 1 percent.The design of the spectrometer of this invention permits inaccuracies tobe reduced to a satisfactory low order with a minimum of difficulty.

The cylindrical symmetry employed facilitates the production of anintense ion beam afiording a high sensitivity in measurements. With thissymmetry, it is comparatively easy to predict the potential distributionin the spectrometer tube and maintain uniform fields. Cylindrical designsimplifies production problems in apparatus requiring preciseconstruction. High grade insulation is readily provided to permit highaccelerating voltages to be used. However, it will be apparent that theinvention is not limited to a cylindrical construction as shown, but isequally applicable to a non-symmetrical construction as in other knowntypes of mass spectrometers wherein With time the ion generator is atone end of the evacuated envelope This application. is acontinuation-in-part of my application Serial No. 262,725, filedDecember 21, 1951.

I claim:

1. Apparatus for determining the atomic mass of a component of a mixtureand the abundance of that component of the mixture comprising means forsubjecting said mixture to a very brief ionizing impulse to generate agroup of ions of said mixture, a plate electrode, means for establishingan electric field of predetermined distribution and magnitude betweenthe area of generation of said ions and said plate electrode to impingesaid ions upon said plate electrode at a fixed distance from thearea ofgeneration, means for delaying a pulse derived from said ionizingimpulse for a predetermined time interval, means for utilizing saiddelayed pulse to gate the detection of said ions by an ion detectingcircuit, the delayed interval in relation to the field parameters beingdeterminative of the atomic mass of the gated ions, means for measuringa function of the abundance of ions so gated, cyclical means wherebysaid steps are repeated rapidly to accumulate the gating effect, andadditional means for measuring the effect of a large number of saidrepetitions.

2. The invention according to claim 1 and means whereby said fieldmagnitude is periodically varied through a predetermined range so thatthe distribution of atomic masses may be determined.

3. The invention according to claim 1 and means whereby the delayinterval is periodically varied through a predetermined range so thatthe distribution of atomic masses may be determined.

4. The invention according to claim 1, said gating means comprisingv aplate electrode circuit for transmitting signal pulses produced byimpingement of ions upon said plate electrode, and a coincidence devicehaving means for producing an output only upon coincidence of saidsignal pulses and said delayed pulses, said measuring means beingconnected to the output of said coincidence device.

5. The invention according to claim 4, said coincidence device havingrespective input circuits for said signal pulses and for said delayedpulses, said input circuits having a time constant not greater than theindividual duration of said pulses.

6. The invention according to claim 5, said coincidence device having anoutput circuit with a much greater time constant than the individualduration of said pulses.

7. The invention according to claim 6, said coincidence. devicecomprising a balanced input circuit for one of said sets of pulses, saidbalanced circuit comprising two branches connected at a common point toa pulse circuit, each branch comprising two capacitors in series with arectifier diode in series between them, said rectifier diodes beingsimilarly oriented, an impedance between the common point of said twobranches and ground, means for feeding the other of said sets of pulsesinto the opposite end of one of said branches, and impedance betweensaid opposite end and ground, and detecting circuit means connectedacross said branches on respectively opposite sides of said diodes, saidbranch circuits constituting the input circuit and said detectingcircuit constituting the output circuit of the coincidence device.

8. The invention according to claim 7, said detecting circuit comprisinga balanced circuit having two further branches of relatively long timeconstant for balancing 15 the outputs of said two first branches underconditions of non-coincidence of input pulses.

9. The invention according to claim 8 and means for adjusting said twofurther branches to obtain initial balance.

10. Apparatus for mass spectrometry comprising an evacuable gasimpervious envelope for receiving a sample for analysis enclosing aplurality of spaced electrodes including a source generator of ionizingelectrons having a grid anode and a plate electrode separated in space,means for pulsing the source generator of ionizing electrons, means foraccelerating ions originating in the near region of the grid anodetoward the plate electrode, means for collecting secondary electronsemitted from the plate electrode by the impact of ions, means for pulsecontrolling delayed in time the passage of secondary electrons from theplate electrode to the collecting electrode, and means for detecting thearrival of gated secondary electrons at the collecting electrode.

11. Apparatus for determining the mass of ions in a material in gaseousform, comprising means for subjecting said material to a very briefionizing impulse to generate a group of ions of said material, means forproducing an electric field of predetermined intensity between saidgenerating means and a plate electrode located at a fixed distance fromsaid generating means for accelerating said group of ions towards saidplate to eject secondary electrons therefrom, a collecting grid forcollecting ejected secondary electrons from said plate, detecting meansassociated with said collector grid for detecting secondary electronsreceived thereby, a gating grid between said plate and said collectorgrid normally biased to prevent passage of secondary electronstherethrough, and means for pulsing said gating grid with a biasingvoltage of such value as to permit passage of secondary electronstherethrough to said collector grid, means for deriving biasing pulsesfrom said ionizing impulses at a predetermined time delay with respectto said ionizing impulses.

12. The invention according to the preceding claim including means forvarying cyclically the field intensity.

13. The invention according to claim 11 including additional grid meansin said field, means for supplying electric potentials to said gridmeans to efiect the distribution of said field, the spacing andpotential of said additional grid means being such that all ions ofequal mass originating in the area of generation have the same time offlight between any point in said area and said plate electrode, thepotential function defining the distribution of said field intensitybeing substantially parabolic.

14. Apparatus for mass spectrometry comprising an evacuable gasimpervious envelope for receiving a sample for analysis enclosing aplurality of spaced electrodes including a source generator of pulsedionizing electrons, a grid anode and a plate electrode for receivingions spaced from said generator, means for producing an ion acceleratingelectric field between the grid anode and the plate electrode, thearrangement and geometry of the electrodes being such as to produce atime-focusing field distribution whereby ions of a given mass-charge atdifferent locations in the vicinity of said generator are accelerated bysaid field toward said plate electrode so as to arrive at said plateelectrode simultaneously, and means for detecting the arrival of ions atsaid plate electrode.

15. The invention according to claim 14, said detecting means comprisinga collector electrode for receiving secondary electrons ejected fromsaid plate electrode by the impact of ions, a gating grid between saidplate and said collector electrode normally biased to prevent passage ofsecondary electrons, and means for pulsing said gating grid with agating pulse of such magnitude as to permit passage of secondaryelectrons at a definite time interval subsequent to said first pulse.

16. Apparatus for mass spectrometry comprising an evacuable gasimpervious envelope for receiving a sample for analysis enclosing aplurality of spaced electrodes including a source generator of pulsedionizing electrons, a grid anode, a focusing grid electrode spaced apreselected distance from the grid anode, and a plate electrode forreceiving ions spaced at a further distance from said grid anode, meansfor producing an ion accelerating electric field between the grid anodeand the plate electrode, the arrangement and geometry of the electrodebeing such as to produce a time-focusing field distribution whereby ionsof a given mass-charge at different locations in the vicinity of saidgenerator are accelerated by said field toward said plate electrode soas to arrive at said plate electrode simultaneously, and means fordetecting the arrival of ions at said plate electrode.

References Cited in the file of this patent UNITED STATES PATENTS2,582,216 Koppius Jan. 15, 1952 2,642,535 Schroeder June 16, 19532,685,035 Wiley July 27, 1954 2,691,108 Berry Oct. 5, 1954 OTHERREFERENCES A Pulsed Mass Spectrometer with Time Dispersion, by Wolff andStephens, published in The Review of Scientific Instruments, vol. 24,No. 8, August 1953, pages 616, 617.

